Dry deposition of peroxyacetyl nitrate (PAN): Determination of its deposition velocitiy at night from measurements of the atmospheric PAN and 222 Rn concentration gradient

During the field campaign POPCORN (Photooxidant Formation by Plant Emitted Compounds and OH‐Radicals in North‐Eastern Germany) in August 1994 we measured the nighttime deposition velocities of PAN above a corn field. These are the first absolute measurements of PAN deposition velocities in the field. The deposition velocities were derived using a novel method, which uses measurements of the gradients of PAN and 222Rn and of the emission rates of 222Rn from the soil. A unique data set of about 250 field measurements of the PAN deposition velocity at night was thus obtained. The deposition velocity at night proved to be highly variable with an average of 0.54 cm/s and a standard deviation of 0.94 cm/s. Recent presumptions by Shepson et al. [1992] that the PAN deposition velocity is strongly reduced with increasing relative humidity could not be confirmed by our measurements.